The present invention relates generally to concentrators that allow different computers to communicate over a data network and, more particularly, to a "dumb" multiple access unit concentrator for a local area network, such as a token ring network. Such concentrators are generally well known and are described, for example, in the Institute of Electronics and Electrical Engineers ("IEEE") Standards for Local Area Networks: Token Ring Access Method and Physical Layer Specifications (Approved Jun. 2, 1989; Std 802.5-1989).
The IEEE Standard discusses the compatible interconnection of dam processing equipment, via a local area network, using the token-passing ring access method. A token ring consists of a set of stations serially connected by a transmission medium, such as, for example, a twisted pair of conductors, coaxial cable, or optical fiber. As set forth in the IEEE Standard:
Information is transferred sequentially, bit by bit, from one active station to the next. Each station generally regenerates and repeats each bit and serves as the means for attaching one or more devices (terminals, work stations) to the ring for the purpose of communicating with other devices on the network. A given station (the one that has access to the medium) transfers information onto the ring, where the information circulates from one station to the next. The addressed destination station(s) copies the information as it passes. Finally, the station that transmitted the information effectively removes the information from the ring.
A station gains the right to transmit its information onto the medium when it detects a token passing on the medium. The token is a control signal comprised of a unique signalling sequence that circulates on the medium following each information transfer. Any station, upon detecting an appropriate token, may capture the token by modifying it to a SFS [start-of-frame sequence] and appending appropriate control and status fields, address fields, information fields, FCS [frame check sequence] and EFS [end-of-frame sequence].
Id. at 20-21. Prior concentrators include, for example, devices such as that shown in U.S. Pat. No. 5,132,832. The patent describes a token ring concentrator for a series of computers having data ports. As disclosed in the abstract of the patent, circuits are disclosed that provide a global reset capability for all of the ports. The circuitry includes transistors that function to both provide a path for the global reset pulse to the reset coils and to isolate each reset coil in a port from the reset coils of the other ports. Other circuitry is shown for preventing activation of the global reset function from affecting an active port.
Some available concentrators may not reliably set, or insert, a computer into a token ring network or reliably reset, or withdraw, the computer from the network. Other concentrators are time consuming or expensive to manufacture. Still others may unexpectedly move between inserting and withdrawing the computer upon being subjected to sudden movement or a mechanical shock.